The present invention relates to a display device which generates a holographic reconstruction in a relatively large reconstruction space (volume of view) with the help of a multiple compound image of a spatial light modulator (SLM).
Generally, visual display devices serve for users to watch, for example, two-dimensional video information, computer-generated stereo images or holographically generated information of a three-dimensional scene.
In addition to display devices where a generated image or a reconstructed 3D scene can be viewed directly, i.e. without optical magnification, as a real image, there are a number of display devices known in the art where a virtually generated image is viewed. Such display devices are also known as virtual image displays (VID).
Displays with virtual image generation are often used in display devices which are located close to the eyes, so-called near-to-eye displays. Such display devices are known for example as head-mounted displays (HMD) or helmet-mounted displays (HMD). They are also often referred to as data glasses'.
Head-mounted displays (HMD) are display devices which are put on by the users like glasses or which are worn on their head similar to a helmet. They can be made to serve one (monocular HMD) or both eyes of an observer (binocular HMD).
The invention shall not be limited to head-mounted displays. It also relates to any stationary or portable display device where the observer views the magnified representation of at least one display panel, or the magnified representation of a three-dimensional scene as reconstructed by a display device, through magnifying lenses which are situated near the eye (ocular). Such a holographic display device resembling an electronic ‘raree show’ shall be referred to as ‘hocular’ below. In contrast to an HMD, devices of this kind do not make such great demands on size and weight. They can preferably be used wherever 3D data sets with great spatial depth shall be viewed over long periods without causing fatigue. This is for example the case in medical equipment where three-dimensional computed tomography, magnetic resonance tomography, confocal microscopy or ultrasonic image data are watched.
The invention can further be applied to display devices which are situated in the observer's line of sight as front-view displays and which also generate a magnified virtual or real image (head-up displays).
Moreover, the invention can be applied to display devices which generate a magnified real image.
If not otherwise mentioned explicitly, the following explanations shall generally apply to display devices with generation of virtual images, such as head-mounted displays or hocular displays. To keep things simple, the term HMD will be used solely hereinafter, but devices with magnified real image generation shall not be excluded.
In an HMD, the observer can be shielded completely from the real environment, so that they only perceive the displayed information in the form of so-called virtual reality (VR). In other devices, the real environment is superimposed with the displayed information, for example through semi-transmissive mirrors, so that an augmented reality (AR) is created.
In a hocular device, the real environment can for example include a microscopic image of a real object, said image being generated through an additional optical path of the microscope. Some devices allow the degree of superimposition to be affected by controllable means. Fully shielded devices can also have at least one camera which is located near the eyes. Superimposing the camera image with the information to be displayed can also enable AR applications.
Various types of head-mounted displays, including glasses-type and helmet-type, have been disclosed in a number of documents.
A HMD device for the stereoscopic representation of 3D scenes is known, for example, from document US 2009/0180194 A1.
A display device in the form of an HMD for the holographic reconstruction of three-dimensional scenes is known for example from document WO 2008/071588 A1 filed by the applicant. In such an HMD, a 3D scene can only be reconstructed in a small volume of view. The viewing window (VW), also referred to as ‘sweet spot’, which is created in this device only has about the diameter of an eye pupil. The viewing window is not typically formed by a real aperture but is only generated virtually.
If the holographic information is encoded one-dimensionally on the SLM, the generated viewing window has an incoherent and a coherent direction. The coherent direction is defined by the distance between two diffraction orders which are generated by the effective grid of the modulator cells. The effective grid is the matrix of the SLM as perceived by the observer eye. The holographic reconstruction can be viewed between the two selected diffraction orders. Their distance must be larger than the diameter of the pupil of the observer eye so to prevent double images. In the case of vertical encoding the diffraction orders are generated in the horizontal direction. The size of the viewing window in the incoherent direction is defined by the angular spectrum of the illuminating light.
In order to avoid active tracking of the viewing window, it should be substantially larger than the eye pupil, for example 15 mm×15 mm. This requires a small pitch of the spatial light modulators that are used in the system, so that the large diffraction angles which are necessary for this can be created. A large light modulator with a high resolution is thus necessary to satisfy this condition while at the same time realising a large field of sight.
The observer eye pupil should normally lie between two diffraction orders in the viewing window of a holographic HMD.
In order to be able to watch the generated reconstruction at a given distance and with a given apex angle at a typical resolution of the human eye at high quality and brightness, an HMD device should have a spatial light modulator (SLM) with a very large number of modulator cells (pixels). This is particularly important where the modulator cells of the SLM shall only be encoded one-dimensionally. Small SLMs which are used in HMDs have a modulator cell matrix of <3*10^6 modulator cells, for example. To be able to provide a larger volume of view for the reconstruction to be generated, an SLM should have a modulator cell matrix of >20*10^6 modulator cells.